Control system for multiple-effect evaporators



oRs

April 6, 1965 s. G. DUKELOW I' CONTROL SYSTEM FOR MULTIPLE-EFFT EVAPORATFiled Sept. 23. 1960 SAMUEL G. DUKELOW A ORNEY United States Patent O3,176,756 CONTROL SYSTEM FR MULTlPLE-EFFECT EVAPRATORS Samuel G.Dukelow, San Mateo, Calif., assigner to Bailey Meter Company, acorporation of Delaware Filed Sept. 23, 1960, Ser. No. 53,107 9 Claims.(Cl. 159-44) My invention relates to a control system for multipleeffeotevaporators used in the pulp industry for the concentration of thinblack liquor to thick black liquor.

As Well known, if the black liquor was discarded from the process afterbeing washed from the pulp it would be necessary to replace all of thechemicals charged to the digester and the cost of pulp would Ibeprohibitive. Hence, economy dictates the recovery of the chemicals ascompletely as possible. The first step in the recovery process is theconcentration of the thin, or weak as it is sometimes called, fblackliquor to heavy lblack liquor suitable for further processing in adirect contact evaporator and recovery furnace.

Liquor received from the washers usually contains from 12 to `15% solidsconsisting of spent chemicals and the non-cellulose constituents of theWood. After concentration in the evaporator system the solids content isordinarily in the order of 45 to 55%. Essentially the Concentrationprocess in the evaporator system consists in boiling oif a considerableportion of the water found inthe Weak liquor. While such boiling oil canbe accomplished in a single evaporator economy dictates the use ofseveral evaporators arranged in series or cascade as it may be called.With this arrangement live stream as theheating medium is admitted tothe rst evaporator in the series and the vapor generated in thisevaporator then becomes the steam supply for the next evaporator and soon. The thin black liquor, having the lowest boiling point, is fed tothe last evaporator in the series and passes in contra flow relation tothe steam through the system.` Insuch a multiple-effect arrangement onepound of steam admitted to the first evaporator will evaporateapproximately one pound of water and the pound of vapor so formed willevaporate approximately one pound of water in the second evaporator andso on down the line resulting` in a material increase in steam economyover a` single evaporator arrangement.

As evaporators or effects are added a point is reached where a Workingtemperature difference between liquor and vapor ceases to exist and theliquor in the evaporator in place of boiling merely simmers. For thisreason the effects or evaporators are usually limited to between and 7.`In contemporary practice saturated steam is admitted to the lirst efecftat about 15 to 20 p.s.i.g. and the vapor formed in the last effectdischargesto a condenser operating at about 25 to 27" of mercury vacuum;the exact vacuum depending upon the temperature and quantity of coolingwater available. p

While the 'multiple-effect evaporator system has materially increasedthe economy of chemical recovery in the pulp industry it has presented adifficult control problem because of the long time constants and smallstorage capacities involved. Without proper control, changes in liquorfeed iiow, solids concentration, steam conditions, cleanliness of heatexchanger surfaces and the like result in an output of black liquor ofnon-uniform concentration. Furthermore because of the long timeconstants, an evaporator system once upset, may require lice hoursV oftrial and error adjustment Ibefore it is again stabilized.

ln an application tiled in the United States Patent Oilice on August 17,1960, Serial No. 50,269, George L. Harman discloses a control system fora multiple-effect evaporator. In the Harman system, steam demand isdetermined in part from the rate of ilow of thin liquor to thelastefect. Such a control system is satisfactory for a straight throughput system. When, however, the how of liquor is detoured through anauxiliary process as by being temporarily detained in a soap separationtank a fixed determination of steam demand from thin liquor flow mayresult in a non-uniform heavy black liquor. The Harman control incorrecting for this nonuniformity may upset the entire evaporator systemwhich may necessitate the system being taken off Automatic and asheretofore mentioned require hours of trial and error adjustment on thepart of the operators before stability is restored. My invention isparticularly directed to a control `system which compensates for theintroduction oi auxiliary processes in multiple-effect evaporators usedin the concentration of thin black liquor to heavy black liquor.

In the drawing: y

FIG. 1 shows in diagrammatic form my invention applied to a typicalmultiple-eifect evaporator system incorporating an auxiliary process. t

Referring to FIG. l I show,.for purposes of illustration, amultiple-elfect system consisting of evaportors L5. inclusive. Thinblack liquor is lfed to evaporators 4 and 5 in parallel from a storagetank 6 by means of a pump 7. I have shown thin liquor fed to evaporators4 and 5 in parallel as this represents a practicefrequently used.

The effluent from evaporator 5 is discharged through a pipe S and forcedinto evaporator 3 through pipe 9 by pump lil. The effluent fromevaporator 4 is likewise fed to evaporator` 3 through pipe 9 by means ofa pump 11. The liquor after passing through evaporator 3 is divertedthrough a soap separation tank 6l) for the intermittent or continuousremoval of tall oil. This separation is essentially a settling processwhere certain ingredients in the liquor found particularly in theso-called Kraft process and known collectively as tall -oil areremoved.` `After leaving the tank oil, the liquor is fed by a. pump 12to evaporator 2, thence fed by a pump 13 .to evaporator 1 whence it isdischarged through pipe i4 to a heavy black liquor storage tank l5.

Steam as a heating medium is supplied evaporator 1 through a pipe i6from any suita'ble source such as the extraction stage of a turbine,pressure reducing valve or the like (not shown) and preferably atconstant pressure. As heretofore noted, in contemporary practice steamis supplied the system at about l5 to 20 p.s.i.g. saturated. Thecondensate of the steam supplied evaporator l is discharged through apipe 17. This condensate may be returned to a steam generator or wasted.

As well known in :the art the decision =to return or waste is frequentlyautomatically determined from a conductivity `sensitive control, anincrease in conductivity indicating contamination of the condensate bythe liquor rendering it unfit for recycling through .the steamgenerator.

The heat of vaporization of the steam fed to evaporator 1 is transferredto the liquor passing therethrough causing the liquor to boil. The vaporso formed is discharged through a pipe 1S to evaporator 2 wherein itforms the snc/aree heating medium for boiling the liquor passingtherethrough. This process is repeated in evaporators 3, 4 and 5, thevapor generated in evaporator 2 discharging through a pipe lie -toevaporator 3, the vapor generated therein discharging through a pipe Ztlto evaporator 4, the vapor generated therein discharging through a pipeZ1. to evaporator 5. The vapor discharged from evaporator 5 passes to acondenser 22 supplied with water through a pipe 23. Specific millconditions determine the type of condenser used, that is, whether it isa jet, or barometric as it is sometimes called, or a surface type.

The condensate from the effects 2, 3, 4 and 5 is dis- V charged throughpipes 2d, Z5, 26 and Z7 respectively.

rl'his condensate may be wasted or reused depending upon millconditions, due care being taken in regard to contamination by theliquor as outlined with respect to evaporator `ll. In some systems thecondensate from one effect is flashed to the next succeeding effectimproving the economy of operation.

I have purposely illustrated a simplified multiple-effect evaporatorsystem to provide an adequate basis for the description of my inventionto follow. As will be apparent to those skilled in the art, ordinarilyauxiliary devices are incorporated in a multiple-effect evaporatorsystem such as catchalls in the vapor outiiow pipe from each evaporatorwhich serve to refine the separation of vapor from liquor, condensatetraps and the like. As such auxiiiary devices are not pertinent to anunderstanding of my invention such have not been illustrated.

The control embodying my invention as shown in FIG. 1 is of the typecommonly known as, pneumatically operated Vfor the reason thatcompressed air is used as the operating medium. However, it will beapparent as the description proceeds that my invention may as readily beincorporated in an electric or hydraulic control. i have chosen to showa pneumatically operated control for the reason that the componentsmaking up the system are well known in the art and their operationreadily understood.

In accordance with my invention the rate of steam flow 1 the to firsteffect is maintained in desired proportion to the rate of liquor fed theevaporator following the auxiliary process, the proportionality beingtemporarily modified in accordance with changes in the` total rate offlow of virgin liquor to the system. I have found that such a controlmaintains the evaporator system stable notwithstanding changes in thevirgin feed rate or by the inclusion of an auxiliary process such as thesoap separation process. Further, I have found that such a controlsystem produces a uniform heavy black liquor, other factors remainingconstant, however, my invention further comprehends automaticallyreadjusting the desired proportionality between liquor flow and steamflow to maintain a predetermined specific gravity of the heavy blackliquor thus correcting for the effect changes in such other factorsmight otherwise have.

There' is shown in the drawing a constant flow control of the thinliquor feed to the evaporator system with automatic ratioing of the feedamong the evaporators, as thin liquor is fed to more than oneevaporator. Flow transmitter 2S establishes a pneumatic loading pressureproportional to the total feed of the thin liquor to the system. I haveshown this iiow transmitter and other transmitters used in my controlschematically as such transmitters may be of any one of several typesavailable. The loading pressure established by transmitter 2S,proportional to the total feed of thin liquor to the system is introdcedthrough pipe 28A into the B chamber of a relay 29 having proportionalplus reset action. The relay 29 may, for example, be of the typeillustrated and described in United States Patent 2,805,678 issued toMichael Panich on September 10, 1957. I have further shown the mechanismof a similar relay 4t?, to which reference will be made later, indiagrammatic form.

into chamber A of relay Z9 is introduced a manually adjustable loadingpressure which establishes Y the set point of the iow control, that is,establishes the demand rate of virgin feed to the evaporator system.When the loading pressures introduced into chambers A and B are equal orstand in predetermined relation to each other the actual rate of feedwill be equal to the demand rate of feed. A convenient means ofadjusting the set point loading pressure may be provided by a selectorstation such as shown at 36 being of the type illustrated and describedin United States Patent 2,747,595 issued to P. S. Dickey on May Z9,1956.

The output pressure at D of relay 29 changes in proportion to changes inthe difference between the loading pressures in chamber A and B and byvirtue of the res ricted connection to chamber C continues to change ata slow rate so long as the pressures in chambers Arand B are unequal ordepart from a predetermined relation. Thus the relay 7,59 is spoken ofin the art as a proportional plus reset type in that it serves to causeimmediate changes proportional to the amount of departure from a desiredvalue and thereafter a continuing change until the actual value isrestored to the desired value.

Tie output pressure at D of relay Z9 after passing through selectorstation 3@ is transmitted through a pipe 31 to a diaphragm operatedvalve S2 regulating the rate of thin liquor feed to evaporator S.

The loading pressure established by the transmitter 25 is, as shown,also transmitted through a pipe 23B to the chamber A of a relay similarto the relay 29. Into the chamber E of this relay is introduced aloading pressure proportional to the rate of flow of thin liquor toevaporator generated by a iiow transmitter 34. With respect to relay 33the loading pressure generated by iiow transmitter 23 may be consideredas establishing the set point of the constant flow control regulatingthe liquor feed to evaporator 4, as the output pressure established at Dof relay 35, transmitted through selector station 35, is effective forpositioning a diaphragm operated valve For any total rate of liquorfeed, the ratio betweeri the rates of feed to evaporators 4 and 5 may beadjusted by means of a relay 36A and a manual loader 37. The relay 36Ais in general similar to relay 29 but is provided with a bellows 38 forremote adjustment of the proportional band as illustrated and describedmore particularly in United States Patent 2,743,710 issued to Jack F.Shannon on May 1, 1956. I have further shown the mechanism of a similarrelay 49, to which reference will be made later, in diagrammatic form.More particularly the ratio between the input pressure at A'and theoutput pressure at D of relay 36A is varied in accordance with theloading pressure in bellows 38 established by manual loader 37. Thus itwill be evident that a one to one ratio between the flows to evaporators4 and 5 may be established or any other desired ratio established byadjusting the proportionality between the input and output pressuresof'relay 36A.

The selector stations 30 and 3S serve primarily as a means of readilytransferring the constant flow control I have described from Automaticto Remote Manual or Hand control as it is frequently called. Eachselector station is provided with a hand adjustable knob which when thecontrol is on Remote Manual establishes the loading pressure forpositioning the final control element which with respect to selectorstation 30 is the valve 32 and with respect to the selector station 35is the valve 36.

ln accordance with my invention the rate of steam flow to the firsteffect is maintained in desired proportion to the rate of flow of liquorto the evaporator following the auxiliary process. Thus in theembodiment of my invention shown in the drawing the rate of steam liowis maintained in proportion to the rate of flow of liquor to evaporator2. I have found that this liquor flow more nearly establishes the rate`of steam ilow required to produce a uniform heavy blaekliquor than doesthe rate of ow of thin liquorto the last etect.

In thel drawing I show"`a constant iow control of the` steam flow, theset point of which is adjusted from liquor flow to evaporator 2. Aloading pressure proi portional to the liquor llow to evaporator 2established `by a ow `transmitter 61 is normally relayed through a relay62 into the A chamber of relay 40 which as previously mentioned isprovided with proportional plus reset action. Into the B chamber of thisrelay the loading pressure generated by a ilow transmitter- 41, asmodified by relay 49, is introduced.` Thus by virtue of relay 40 therate of steam ow is normally maintained in predetermined ratio to theliquor ow to evaporator 2. The steam ilow constant flow control loop isshown as provided with a selector station 42 and a diaphragm operatedflow control valve 43 as the nal control element. Changes in liquor flowwill effect immediate and pro portionate changes in steam ow therebyVmaintaining the heat input to the system as required to produce anoutflow of liquor from the system of uniform concentration. Changes insteam requirement occasioned by changes in liquor flow are made beforesuch changes cause an upset in the system.`

The loading pressure established by iiow transmitter 61 is introducedinto the C chamber of relay 62 and is normally reproduced in the Dchamber and transmitted to relay 40 through a pipe 63. The loadingpressure 'established by flow transmitter 28 is introduced through pipe2SC directly into the A chamber of relay 62 and through an adjustablerestrictor 64 into the B chamber. Changes in yirgin feed to the lastevaporator will thus cause immediate and proportionate changes in steamflow as the pressure at D of relay 62 is proportional to the sum of thepressures introduced at A and C. 'I'his change in steam flow graduallydecays or is wiped out as the pressure in chamber'B becomes equal tothat in chamber A as the pressure 1n these two chambers act inopposition. By proper adjustnient of the restrictor 64 and theproportional band of relay 62 the temporary change in steam ow producedby relay 62 will maintain stability of the evaporator system and theuniformity of the heavy black liquor.

Further in accordance with my invention the constant steam llow controlmay be trimmed or readjusted automatically in accordance with changes indensity or coricentration of the black liquor outflow from theevaporator system. I show by wa'y of example the concentration ordensity of the black liquordetermined by comparing theboilingtemperature of the black liquor to the temperature ofboilingwater at the pressure of the black liquor. The dilterence betweenthese two temperatures is a measure of densityor concentration of theliquor. Referring to the drawing I therein show a condenser 44 suppliedwith steam from pipe 16 through an adjustable restriction 45. Thecondenser 44 is maintained at the same pressure as the black liquor bya` vent pipe 46 `connected to the vapor space in evaporator 1.` Thearrangement therefore provides a means for maintaining a water sample atsaturation temperature at the same pressure as the black liquor inevaporator 1. i

The black liquor leaving evaporator 1 is at boiling temperature,` hencea differential temperature device such as shown `schematically at 47responsive to the temperature of the water in condenser 44 and to thetemperature of the black liquor in pipe 14 may be utilized to generate aloading pressure proportional to the density of the black liquor. Thisloading pressure after passing through a selector station 48s`erves toadjust the proportional band of relay 49 disposed in the loading linebetween ow transmitter 41 and relay 40. Changes in liquor density willthereby cause a change in the proportion between the actual rate ofsteam iiow and loading pressure admitted to chamber B of relay 40. Thusfor a given rate of liquor flow a new rate of steam flow will bemaintained as re- (t3 quired to hold a given density or concentration ofthe black liquor leaving the system. The selector station 48 provides ameans for manual-ly adjusting the ratio between liquor and steam inputsif desired.

As well known the stability of operation is improved by maintaining apredeterminel absolute pressure in condenser 22, this in some cases maybe accomplished by a single element control wherein an absolute pressuremeter such as diagrammatically illustrated at S1 establishes a loadingpressure proportional to the absolute pressure in the condenser 22 whicheither directly, or indirectly through a relay to give proportional plusreset action (not shown), controls the rate of cooling water to thecondenser by positining a diaphragm valve 52.

Certain types of surface condensers possess undesirable time lagcharacteristics. Where such a condenser is used it is desirable toprovide .la two element control such as I have shown which regulates theflow of cooling water to maintain a predetermined temperature of thecooling water in the condenser. The predetermined temperature isadjusted as required to maintain the desired absolute pressure in thecondenser. I have schematically shown such an arrangement in the drawingwherein a temperature transmitter 53 establishes a loading pressureproportional to the temperature of the cooling water in or leaving thecondenser and which through a relay 54 .provides an output pressure forpositioning the valve 52. The set point of this constant temperaturecontrol is adjusted las required to maintain a predetermined absolutepressure in the condenser by introducing the loading pressure generatedby transmitter S1 into the B chamber of relay 54, which as shown, isprovided with proportional plus reset action.

The embodiments of the invention in which an eX- clusive property orprivilege is claimed are denedas follows:

l. In a control system for a multiple-effect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled olf the liquor in one evaporator Iacts as the heatingmedium in the following evaporator and arranged to admit steam as theheating medium to the first evaporator and ythin black liquor to thelast evaporator whereby the black liquor and heating medium pass throughthe system in contra flow relation, the combination comprising; `controlmeans maintaining a constant rate of flow of the steam admitted to thefirst evaporator having an adjustable set point, means responsive to theflow of liquor -at a point in the flow stream between the first and lastevaporators adjusting the set point to maintain a predetermined constantratio between the rate of o'w of liquor at said point and the rate offlow of steam to the first evaporator and means responsive to changes inthe rate of ow of thin black liquor to the last evaporator temporarilymodifying the predetermined constant ratio in accordance witlr changesin the rate of ilow of thin black liquor to the last evaporator.

2. In a control system for a multiple-etect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled ott the liquor in one evaporator acts as the heating mediumin the following evaporator and arranged to admit steam as the heatingmedium to the first evaporator and thin Y black liquor to the lastevaporator whereby the black liquor and heat-ing medium pass through thesystem in contra how relation, thecombination comprising; control meansmaintaining a constant rate of flow of the steam admitted to the firstevaporator having an adjustable set point, means responsive to the ilowof liquor at a point in the flow stream between the rst and lastevaporators adjusting the set point to maintain a predeterminedoonstantratio between the ow `of steam to the first evaporator and theflow of liquor at said point in the flow stream, means measuring riow ofVthin liquor to the last evaporator', means operated by said last namedmeans producing a control eiiect proportional to the rate oi change inthe ilow of thin liquor and means operated by said control eiiectmodifying said predetermined constant ratio in substantially directproportion to the rate of change in the iiow of thin black liquor.

3. In a control system tor a multiple-effect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled `oil the liquor in one evaporator acts as the heatingmedium in the following evaporator and arranged to admit steam as theheating medium to the iirst evaporator and to admit thin black liquor tothe last evaporator whereby the black liquor and heating medium passthrough the system in contra tiow reaction and wherein the liquor isdiverted through a soap separation tank in passing from one evaporatorto another, the combination comprising; control means maintaining aconstant rate of flow of the steam admitted to the iirst evaporatorhaving an adjustable set point, means responsive to the tiow of liquorleaving the soap separation tank adjusting the set point to maintain apredetermined ratio between the flow of steam to the first evaporatorand the How of liquor leaving the soap separation tank, means measuringthe flow or thin liquor to the last evaporator, means operated by saidlast named means producing a control effect proportional to the rate ofchange in the iiow of thin liquor and means operated by said controleffect modifying said predetermined ratio to produce a temporary changein the 'low of steam to the rst evaporator in substantially directproportion to the rate of change in flow of thin 'liquor to the lastevaporator.

tem in contra iiow relation, the combination comprising;

means producing a control eiieet corresponding to changes in the iiow ofthin liquor to the system and means operated by said control electtemporarily changing the iiow of steam to the rst evaporator insubstantially direct proportion to changes in the ow of thin liquor tothe last evaporator.

5. VIn a control system for a multiple-eiiect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled oil"r the liquor in one evaporator acts as the heatingmedium in the following evaporator and arranged to admit steam as theheating medium to the first evaporator and thin black liquor to the lastevaporator whereby the black liquor' and heating medium pass through thesystem in contra liow relation, the combination comprising; constantiiow control means of the steam admitted to the iirst evaporator havingan adjustable set point and means eiieoting a temporary adjustment ofsaid set point in substantially direct proportion with changes in therate of ilow of thin liquor to the last eiect.

6. In a control system fora multiple-eiiect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled off the liquor in one evaporator acts as the heating mediumin the following evaporator and arranged to admit steam as the heatingmedium to the first evaporator and thin black liquor to the lastevaporator whereby the black liquor and heating medium pass through thesystem incontra ow relation, the combination comprisun ing; constantflow control means of the steam admitted to the iirst evaporator havingan adjustable set point, means measuring the flow of thin liquor to thelast evaporator, means operated by said last named means producing acontrol etiect proportional to the rate of change in the iiow of thinliquor and means operated by said control adjusting said set point toproduce a temporary change in the ow of steam to the first evaporator indirect proportion to the rate of change in the iiow of thin liquor tothe last evaporator.

7. In a control system for a multiple-effect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled 01T the liquor in one evaporator acts as the-heating mediumin the following evaporator and arranged to admit steam as the heatingmedium to the first evaporator and to admit thin black liquor to thelast evaporator whereby the black liquor and heating medium pass throughthe system in contra iiow relation, the combination comprising;regulating means of the rate of iiow of steam supplied the system, meansmeasuring the low of liquor at a point in the flow stream between thelirst and last evaporators in the system,k and means operated by saidlast named means operating said regulating means to produce changes inthe rate of iiow of steam in substantially direct proportion to changesin the How of liquor at said point.

8. ln a control system for a multiple-effect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled off the liquor in one evaporator acts as the heating mediumin the following evaporator and arranged to admit steam as the heatingmedium to the iirst evaporator and thin black liquor to the lastevaporator whereby the black liquor and heating medium pass through thesystem in contraliow relation, the combination comprising; `conrol meanshaving an adjustable set point maintaining a constant rate of iiow ofsteam admitted to the first evaporator, means measuring the ow of liquorat a point in the flow stream between the first and last evaporators,means operated by said iiow measuring means generating a control effectcorresponding to the flow of liquor at said point, and means operated bysaid control eiect adjusting the set point of said control means tomaintain a constant ratio between the rate of flow of liquor at saidpoint and the rate of ow of steam to the first evaporator.

9. In a control system for a multiple-effect evaporator system for theconcentration of thin black liquor to heavy black liquor composed ofseveral evaporators arranged in cascade by vapor piping in which thewater boiled oirr the liquor in one evaporator acts as the heatingmedium in the following evaporator and arranged to admit steam as theheating medium to the first evaporator and to admit thin black liquor tothe last evaporator whereby the black liquor and heating medium passthrough the system in contraow relation and wherein the liquor isdetoured through an auxiliary process, in passing `from a givenevaporator to the next downstream evaporator with regard to liquor iow,the combination comprising; means measuring the flow of liquor leavingthe auxiliary process, regulatingmeans of the rate of flow of steam tothe system, and means operated by said measuring means operating saidregulating means to produce changes-.in the rate of tiow of steam insubstantially direct proportion to changes in the llow of liquor leavingthe auxiliary process.

References Cited bythe Examiner UNTED STATES PATENTS 1,996,526 4/35Serpas 159-31 X 2,040,284 5/36 Tell 15s- 44 2,073,825 3/37 Beck et al.159-44 {ther references on following page) 9 10 UNITED STATES PATENTSFOREIGN PATENTS 11/38 Holven 159--44 1,223,627 6/60 France. 11/39Hinckley 159-20 d s/so Stinson 159-44 OTHER REFERENCES 1/55 Reynolds 261114 5 Integrated Control System, A. C. Camp pubhcatlon 5/56 Shannon.Sugar,pp. 26, 27, Iune 1950. 5/56 D- k 7/57 fniegyet al 159 44 X NORMANYUDKOFF, Primm Examiner. 9/ 57 PaliCh- GEORGE D. MITCHELL, CHARLESOCONNELL, 6/58 Lankenau 159-20 10 BENJAMIN BENDETT, Examiners. 5/61Bellinger 262-160

3. IN A CONTROL SYSTEM FOR A MULTIPLE-EFFECT EVAPORATOR SYSTEM FOR THECONCENTRATION OF THIN BLACK LIQUOR TO HEAVY BLACK LIQUOR COMPOSED OFSEVERAL EVAPORATORS ARRANGED IN CASCADE BY VAPOR PIPING IN WHICH THEWATER BOILED OFF THE LIQUOR IN ONE EVAPORATOR ACTS AS THE HEATING MEDIUMIN THE FOLLOWING EVAPORATOR AND ARRANGED TO ADMIT STEAM AS THE HEATINGMEDIUM TO THE FIRST EVAPORATOR AND TO ADMIT THIN BLACK LIQUOR TO THELAST EVAPORATOR WHEREBY THE BLACK LIQUOR AND HEATING MEDIUM PASS THROUGHTHE SYSTEM IN CONTRA FLOW REACTION AND WHEREIN THE LIQUOR IS DIVERTEDTHROUGH A SOAP SEPARATION TANK IN PASSING FROM ONE EVAPORATOR TOANOTHER, THE COMBINATION COMPRISING CONTROL MEANS MAINTAINING A CONSTANTRATE OF FLOW OF THE STEAM ADMITTED TO THE FIRST EVAPORATOR HAVING ANADJUSTABLE SET POINT, MEANS RESPONSIVE TO THE FLOW OF LIQUOR